This invention relates to halftone artwork and, in particular, to a method and apparatus for reproducing such artwork.
Halftone artwork is used in many applications. This type of artwork employs halftone dots (i.e., darkened or black dots) to provide an image. In particular, by changing the size of the dots, different gray levels can be produced so as to simulate the grey levels of the image. The halftone dots are typically situated along a screen direction and the dot spatial frequency and period in the screen direction, called the "screen frequency" f.sub.SCR and "screen period" P.sub.SCR, are kept constant.
In some applications, it is desirable to be able to reproduce halftone artwork. In present day reproduction systems, a scanner typically scans the elemental areas (i.e., "pixels") of the original halftone artwork in orthogonal first and second directions. The scanner operates in these directions at corresponding first and second spatial scanning frequencies f.sub.SCA1, f.sub.SCA2 having associated periods P.sub.SCA1, P.sub.SCA2. These frequencies and, thus, their resultant periods, are usually equal.
As a result of this scanning, digital signals are developed corresponding to the gray level values of the orthogonal array of pixels. These digital signals are then thresholded at a threshold gray level value which typically might be at the mid-point of the gray tone range which extends from 255 (black) to 0 (white). This thresholding provides a corresponding set of thresholded or binary gray level values.
More particularly, each digital signal representing a gray level equal to or above the threshold gray level value, e.g., above 128, results in a corresponding binary signal indicative of a first gray level value, typically a 255 (black) gray level value. Each digital signal representing a gray level value less than the 128 threshold, in turn, results in a corresponding binary signal indicative of a second gray level value, typically a 0 (white) gray level value. The resultant set of binary signals are then used to control a printer. The latter generates reproduction pixels corresponding to the signals and, therefore, to the scanned array of pixels of the original halftone. The printer prints in the orthogonal first and second directions at respective spatial print frequencies f.sub.PRI1, f.sub.PRI2 having corresponding print periods P.sub.PRI1, P.sub.PRI2. Again, these frequencies and thus their respective periods are usually equal.
More specifically, the printer darkens each reproduction pixel whose corresponding binary signal is indicative of a 255 (black) gray level value and leaves blank or undarkened each reproduction pixel whose corresponding binary signal is indicative of a 0 (white) gray level value. These black and white reproduction pixels then combine to produce the reproduction of the halftone original.
While the above method results in reproducing the halftone original with some degree of success, it has been found to generate so called "Moire patterns" in the reproduction. These patterns represent beat patterns of the screen and scan frequencies and are undesirable in most applications.
Accordingly, research efforts have been underway to develop practices for reproducing halftone originals which generate little or no Moire patterns in the reproduction. Copending patent application Ser. No. 241,073, assigned to the same assignee hereof, discloses one such reproduction method. In the case of the method of '073 application, the scanning frequency of the system scanner is selected to have a preselected relationship with respect to the screen frequency of the original. Additionally, an adaptive threshold processing technique is utilized in developing the thresholded binary gray level signals from the gray level digital signals of the scanned pixels.
This processing makes use of a scanning window which enables processing of sub-arrays of the pixel digital signals. The size of the scanning window is appropriately selected to be equal to the screen period translated to the scanning direction. Moreover, the processing is such that the average gray levels of the resultant binary signals associated with a given sub-array of pixels is equal to the average gray level of the corresponding digital signals over the sub-array.
The method of the '073 application has been found to provide a significant reduction in the Moire patterns evidenced in the reproduction. However, efforts are still underway to develop other alternative methods which can also provide reproduction of halftone originals with reduced Moire.
It is, therefore, an object of the present invention to provide a method and apparatus for reproducing halftone originals having a reduced amount of Moire patterns.